The present invention relates to an electronically controllable brake actuation system of the type as it is disclosed in German patent application No. 31 24 755. The pressure source of the prior art brake actuation system includes a pump, a hydraulic accumulator and a pressure fluid supply reservoir. The function of the separating valves and the inlet and outlet valves is performed by multi-position valves or four-way/four-position directional control valves having inlet ports that are connected to the pressure side of the pump or the accumulator, the pressure fluid supply reservoir and each one pressure chamber of the dual-circuit master brake cylinder. The wheel brakes are connected to the outlet ports. During independent braking or pressure increase, the four-way/four-position directional control valve is switched to its first operating position where the wheel brakes are separated from the master brake cylinder and connected to the pressure source. A phase where the pressure is maintained constant is achieved in a second operating position where the wheel brakes are isolated from the master brake cylinder and from the pressure source. Pressure decrease is effected in a third operating position where a connection is provided between the wheel brakes and the unpressurized pressure fluid supply reservoir.
Apart from the comparatively high costs incurred by the use of the sophisticated pressure source, the meterability of braking pressure in the prior art brake actuation system is a disadvantage, especially in the low-pressure range, which is due to the use of the four-way/four-position directional control valves.
Therefore, an object of the present invention is to improve upon an electronically controllable brake actuation system of the above-mentioned type to such effect as to virtually eliminate the shortcomings mentioned with respect to ABS control. More particularly, the objective is to considerably improve the meterability of braking pressure in the low-pressure range and to additionally reduce the total effort and structure.